CDPD:Cellular Digital Packet Data

Daniel Grobe Sachs

Quji Guo

What is CDPD?

• Motivation: Packet data over AMPS– AMPS is unsuited for packet data

• Long call setup times

• Modem handshaking required

– Analog providers have AMPS allocation.• Use AMPS channels to provide data service.

• “Cellular digital packet data”

• Can’t interfere with existing analog service.

– CDPD is cheap: no new spectrum license needed!

Design Goals

• Goals:– Low speed, high latency data service

• Primarily intended for paging and email.

– Provide broadcast and multiple-access service.– Dynamically shared media, always online.– Share channels with AMPS allocation– Transparency to existing AMPS service.

CDPD History

• Standard released Jan, 1995 (v1.1)

• Initially used by police (~1996)

• Wide service availability around 2000– Omnisky, Verizon Wireless, others.

• Covers most US population centers – Champaign-Urbana now covered.– Rural area coverage poor.

CDPD Market

• CDPD is used primarily for– Law enforcement– Handheld/laptop IP access– Main competition: “Wireless Web” phones.

• CDPD costs:– Wireless modems: ~$300 (Omnisky Palm V) – Service: $30-$40 per month (handheld)– $40-$80 per month (laptop)

Omnisky Coverage Map

Source: Omnisky (http://www.omnisky.com)

CDPD Infrastructure

Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”

CDPD - Layering

Physical

MAC

MDLP

SNDCP

IP/CLNP

Physical

Data link

Network

Transport

Application

Network layer CDPD Layer

Connectionless Network Protocol

Subnetwork Dependant Convergence Protocol

Mobile Data Link Protocol

Media Access Control

Physical

CDPD Physical Layer

• 30KHz BW channels, shared with AMPS

• Separate forward and reverse channels– Forward channel is continuous– Reverse channel is multiple access.

• Gaussian Minimum-Shift Keying-GMSK– GMSK compromises between channel

bandwidth and decoder complexity.

• 19.2kbps per channel.

Physical

MAC

MDLP

SNDCP

IP/CLNP

AMPS and CDPD

• CDPD runs alongside AMPS– AMPS system is unaware of CDPD system– CDPD system watches AMPS behavior

• AMPS generally has unused channels.– Blocked calls when all channels are allocated.– 1% block probability => all channels used only

1% of the time.

CDPD Channel Usage

• CDPD uses unused AMPS channels.– Usually are several available.– Each 30KHz channel = 19.2kbps up and down

• CDPD channel hopping.– Forced: AMPS must be vacated within 40ms of

allocation for voice use.– Planned: Regular hops prevent AMPS system

from identifying channel as unusable.

Channel Scanning

• 1. Check signal levels from nearby cells.– Use a list of reference channels distributed by

the CDPD infrastructure to find levels.

• 2. Select cell with best signal.– If non-critical and no cell is significantly better

than current, no handoff is done (hysteresis)

• 3. Scan RF channels in cell for CDPD.– Stop when an acceptable channel is found.

Handoff in CDPD

• Critical handoffs: Must choose new channel– High error rate is observed or BS signal lost.– Received signal strength below a threshold.– Base station does not receive data from mobile.

• Noncritical handoffs– Channel rescan interval expires.– Signal strength changes significantly.

CDPD effects on AMPS

• CDPD logically transparent to AMPS

• Can reduce AMPS service quality– More channel usage => increased interference.– If AMPS system is close to SIR margin, CDPD

can push it below.– Full CDPD usage can push SIR down ~2dB

• 19 channels/cell, Pblock = 0.02, 12.3 Erlangs

– Limiting channels used reduces SIR cost..

Data Transmission Format

• All links are base to mobile.– Mobile to mobile goes through base station.– Full-duplex; separate forward and reverse links.

• Forward link– Continuous transmission by BS

• Reverse link– Shared multiple access for mobiles.– Reverse link activity indicated by BS.

Physical

MAC

MDLP

SNDCP

IP/CLNP

Forward Link Structure

Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”

Reverse Link Structure

Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”

Reverse Link MAC

• Near/Far problem– Mobile may not detect a faraway transmitter.– Base station must report busy status.

• Protocol: – Digital Sense Multiple Access– Nonpersistant: Checks once for busy state.– Slotted: Can only start when BS reports state.

• Similar to Ethernet MAC.

Physical

MAC

MDLP

SNDCP

IP/CLNP

Reverse Link MAC

Source: J. Agostsa et al., “CDPD: Cellular Data Packet Standards and Technoloy”

Reverse Link MAC

• Reverse link idle => can transmit.– Busy status checked before transmission starts..– Continue burst unless error is indicated.– If BS indicates error, assume collision;

exponential backoff is used.

• Reverse link busy:– Delay for a random number of slots.– Check busy status again.

Mobile Data Link Protocol

Physical

MAC

MDLP

SNDCP

IP/CLNP

Source: J. Agostsa et al., “CDPD: Cellular Data Packet Standards and Technoloy”

CDPD - MDLP

• Mobile Data Link Layer Protocol (MDLP)– High-level data link control (HDLC)

• Similar to ISDN HDLC.

– Mobile (M-ES) to Infrastructure (MD-IS)• In this layer, air link and BS become transparent• Connection oriented

– MDLP Frame (message structure) • Address, control field, information field• No checksum; MAC discards incorrect packets.

CDPD - MDLP

• Temporary equipment identifier (TEI)– Identifies destination mobile - virtual address.– Assigned by infrastructure.

• Packet types– Unacknowledged information– Sequenced information

• Sequence number, ack, timeout • Sliding window• Selective rejection supported.

CDPD - SNDCP

• Subnetwork-Dependent Convergence Protocol (SNDCP)– Between IP or CLNP and MDLP– In both mobile and infrastructure (MD-IS)– Segmentation, compression, encryption– Questions:

• Where and how to segment data?

• Where and how to compress data?

Physical

MAC

MDLP

SNDCP

IP/CLNP

CDPD - SNDCP

• Segmentation– Goal: to fit the size of underlying frames– Two type of headers

• Sequenced headers: – For compressed, encrypted, and segmented user data.

• Unnumbered headers: Control information.

– Efficiency consideration (similar to X.25)• Which layer should segment/assemble messages?

• Use “More” indicator to avoid IP fragmentation.

CDPD - SNDCP

• Compression– Header compression

• To send the “Delta” information

– Data compression• V.42bis – a dictionary-based compression

– Which layer should compress data?• Source-dependent compression – higher layer

• Source-independent compression – lower layer

CDPD - Registration

• Low-level protocols ignore authentication.

• Registration and Authentication– M-ES, serving MD-IS, home MD-IS

• Base station (MDBS) has no network function.

– Network Equipment identifier (IP, etc.)– Forwarding database in home MD-IS

• Deregistration– Table maintenance timer

Problems with CDPD

• Limited bandwidth– 19.2kbps shared per channel– Modern applications demand more bandwidth.

• Security:– “Man in the middle” identity theft attack– IP network attacks– Denial of Service attacks easy.

Potential Improvements

• Multichannel / multicarrier transmission– Would allow faster rates with AMPS

compatibility.

• Security Improvements– Secure against “man-in-the-middle” attacks.

• Switch to CDMA/GSM.– Digital cellular services are more able to

accommodate data services.

References

• J. Agosta and T. Russell, CDPD: Cellular Packet Data Standards and Technology, McGraw Hill, 1996.

• Y. Frankel et al., “Security Issues in a CDPD Wireless Network,” IEEE Personal Communications, August 1995, pp. 16-26.

• D. Saha and S. Kay, “Cellular Digital Packet Data Network,” IEEE Transactions on Vehicular Technology, August 1997, pp. 697-706.

• A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach,” IEEE Communication Magazine, June 1999, pp. 152-159.

• A. Salkintzis, “Radio Resource Management in Cellular Digital Packet Data Networks,” IEEE Personal Communications, December 1999, pp. 28-36